Obesity continues to be a significant public health problem in America. While treatments for obesity are often successful in the short term, weight recidivism remains a critical problem. However, one of the factors strongly and consistently related to the maintenance of weight loss is exercise. In order to enhance the ability of exercise to facilitate weight maintenance, the mechanism must be elucidated. Exercise may have a direct effect on weight loss and maintenance by enhancing energy expenditure or indirect effects such as favorably modifying lean body mass and resting metabolic rate and improving psychological functioning. Thus, the primary aim of the proposed study is to assess the mechanism by which exercise facilitates weight maintenance. The proposed hypothesis is that exercise acts by improving psychological functioning thus making it easier for individuals to maintain lifestyle behavior changes. Two hundred and fifty women aged 25-45 and 30-50 pounds over ideal body weight will be recruited to participate in a 24 week behavioral weight loss program that includes diet and exercise. Subjects will be followed for 12 months post-treatment and measures related to exercise's hypothesized mechanisms of action will be assessed at 6 month intervals. Path analysis and structural equation modeling will be used to test the fit of various models depicting hypothesized relationships of exercise to weight maintenance. The proposed analysis allows for testing the relative contribution of direct and indirect effects of independent variables and provides a vehicle for evaluating the proposed mechanism. The outcome of this research will allow for the development of more targeted exercise and weight maintenance interventions. Currently 90 subjects have been enrolled in the protocol and are participating in either the treatment or maintenance phase of the protocol. We anticipate enrolling an additional 90 subjects between 12/1/98 and 11/30/99.